1. Field of the Invention
The invention relates generally to a multi-domain vertical alignment (MVA) liquid crystal display device, and more particularly to a multi-domain vertical alignment liquid crystal display device for improving transmittance.
2. Description of the Related Art
A liquid crystal display device (LCD) is a flat-panel display providing images with the same quality as a cathode ray tube. An LCD is used as a flat-panel display for personal computers, televisions, mobile phones and various office automated devices, and is expected to gain further popularity in the display market.
Generally, the light transmittance of an LCD device is measured by applying an electric field to a liquid crystal injected between two substrates. An LCD device comprises a pair of transparent substrates with a transparent electrode formed inside and a polarizer disposed outside. The liquid crystal material with a negative dielectric anisotropy filled between the two substrates is vertically aligned (VA). The electric field is generated in the direction perpendicular to the substrate when the voltage is applied to the electrode. The long axis of the liquid crystal molecule is rotated in the direction perpendicular to the electric field. Therefore, in the substrates of the vertically aligned type LCD devices, the transmittance is changed by horizontally moving the vertically aligned liquid crystal molecules according to the vertical electric field applied in the vertical direction. Additionally, a multi-domain vertical alignment method (MVA method) exists, wherein liquid crystal molecules rotating in different directions within a pixel are controlled.
Proposed processes for achieving the MVA method include forming a protrusion on a pixel electrode and a common electrode, forming an opening on the pixel electrode and the common electrode, and combinations thereof. In the method of forming the protrusion, when no voltage is applied, the liquid crystal molecules are vertically aligned and slightly inclined according to the shape of the protrusion. Therefore, when voltage is applied, the liquid crystal molecules are inclined toward a predetermined direction. The method of forming the opening is substantially the same as the method of forming the protrusion, wherein the direction of the electric field in the opening is inclined toward the liquid crystal molecules.
However, because it is difficult for voltage to be applied to the protrusion and the opening, transmittance is unfavorably reduced. Accordingly, the overall transmittance is decreased.
Specifically, when the protrusion or the opening is formed, the aperture ratio is deteriorated. It is desirable that the area ratio of the protrusion or the opening is low. However, for example, in a portion where straight openings (hereinafter called the slits) are provided in parallel, the liquid crystal molecules are aligned in a direction perpendicular to the direction in which the slits extend. If the width of each slit is not large enough, the liquid crystal molecules will be unable to align in a direction perpendicular to the direction in which the slits extend. That is, the width of the slit must be larger than a predetermined width.
For example, in a vertically-aligned LCD device method (VA method), viewing angle characteristics are proposed to be improved by dividing each pixel into a plurality of sub-pixel electrodes, directly driving each sub-pixel through a data bus line and a switching element, converting an input image signal, and driving the sub-pixel with a first and a second driving signal corresponding respectively to the hue of a first and a second output. Additionally, a predetermined distance is required, desirably at least three times the thickness of the liquid crystal layer of the pixel, between the slits of the sub-pixel electrodes for regulating the alignment. That is, for an MVA, the aperture ratio of the MVA type LCD panel is decreased when the sub-pixel electrodes is divided by the slit for improving the viewing angle characteristics (as disclosed in the patent reference 1, Jpn. Pat. Appln. Kokai Publication No. 2005-316211).
Further, as shown in FIG. 1a, a circular opening or a protrusion may also be disposed over the pixel electrode or at the center of the pixel over the pixel electrode. The liquid crystal is aligned in a direction shown in FIG. 1a in accordance with the protrusion or the opening of the pixel electrode. Generally, the liquid crystal centers on the opening of the pixel electrode or the protrusion and aligns radially, as shown in the inclination 13 and 14 of the liquid crystal of FIG. 1a. That is, the direction for the liquid crystal alignment changes by 360 degrees around the opening of the pixel electrode or the protrusion. FIG. 6 is a graph depicting a relation between the direction for the liquid crystal alignment and transmittance. When the absorption axis of the linear polarizer makes use of an orthogonal polarizer with an angle of 0 and 90 degrees, the transmittance of the liquid crystal is uniformly divided into eight areas according to the direction of the liquid crystal alignment, i.e., dark, bright, dark, bright, dark, bright, dark, and bright. In FIG. 1a, the inclination 13 of the liquid crystal is a bright area and the inclination 14 of the liquid crystal is a dark area. As a result, since the area with low transmittance becomes equal to half the total area, the entire transmittance is lowered.
A method disclosed in Jpn. Pat. Appln. Kokai Publication No. 2005-106972 (patent reference 2) is proposed to solve such problems by adding dissymmetry molecules to the liquid crystal. However, as the amount of the same material is further increased, the response speed may be decreased. Moreover, though a circular polarizer may be utilized in place of a linear polarizer, undesirably higher costs would also be a result.